Hi Ian, Thanks, I understand the backoff now. And I searched the archive, Tom did mentioned that the --tx-amplitude can not be set too big due to the large PAPR of OFMD. I tried to set the tx-amplitude as 0.2 ～ 0.3, and the observed supported bandwidth is still about 1~1.2MHz on my i7 4core computer. The ping packet loss is 20% ~ 40%.
Actually, I just want to know any performance on such OFDM link has been reported or not, as my benchmark. On Fri, May 18, 2012 at 1:12 AM, Ian Buckley <[email protected]> wrote: > Alex, > I'm not entirely sure you correctly understood Brian's point...he's > referring to "backing off" the Tx gain to ensure that the RF power amp > remains entirely linear throughout the power envelope of the OFDM signal. > The PAPR is directly proportional to the number of carriers employed and is > one of the problematic aspects of OFDM. Most USRP daughter cards (or any > radios) tend to loose linearity as they approach the top end of there gain > range. The GMSK signal however being FSK based has a constellation that > remains on the unit circle and hence has constant amplitude allowing it to > operate well in the non-linear region of an RF amplifier (hence it's > popularity in low cost, high efficiency single chip radios). It's easy > therefore to see why empirically it might appear that GMSK is "out > performing" OFDM here, given that this is a simple PHY demonstration, > rather than an application providing a robust link with appropriate channel > coding. Tom has addressed similar questions as yours in the past here, I > suggest you search the archive, I think you'll find more useful > information. > -Ian > > On May 17, 2012, at 9:00 PM, Alex Zhang wrote: > > What's the EVM of your OFDM signal coming out of the RF daughterboard? >> Are you backing off enough to allow for OFDM's high PAPR to remain in >> the linear region? >> >> I think backoff time is enough, as I actually did not see the conflict > occurs. > > >> GMSK has the nice advantage of being constant envelope, so compression >> doesn't matter. OFDM, on the other hand, doesn't get that luxury and, >> in fact, requires a significant backoff. > > > -- Alex, *Dreams can come true – just believe.*
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